1. Field of the Invention
The present invention relates to composite laminates and to methods for manufacturing the same. In particular, the present invention relates to a composite laminate which shrinks less during firing and to a method for manufacturing the same.
2. Description of the Related Art
In recent years, there have been advances in the reduction in size and weight of chip components. Reduction in size and weight is also required for circuit boards for mounting the chip components. Glass ceramic multilayered circuit boards are useful to meet this need because the glass ceramic multilayered circuit boards allow high-density wiring and reduction in thickness, resulting in reduction in size and weight.
Glass ceramic multilayered circuit boards are generally formed by a sintering process, and they shrink during sintering in a direction perpendicular to the main faces of the boards (longitudinal shrinkage) and in a direction parallel to the main faces (transverse shrinkage). Thus, the current glass ceramic multilayered circuit boards have dimensional variations of approximately xc2x10.5%. Glass ceramic multilayered circuit boards having cavities for mounting necessary electronic components exhibit noticeable variations.
Japanese Unexamined Patent Application Publication Nos. 5-102666 and 7-330445 disclose methods for making glass ceramic multilayered circuit boards having high dimensional accuracy. Also, Japanese Unexamined Patent Application Publication No. 6-329476 discloses a method for making a glass ceramic multilayered circuit board having cavities. In each method, green sheets which cannot be sintered at the sintering temperature of a glass ceramic compact are laminated on one side or two sides of the glass ceramic compact, and powdered layers of the green sheets are removed after firing.
In such a process, an additional process is required to remove the powdered layers. Moreover, it is difficult to simultaneously fire conductive films, which are preliminarily formed on an unsintered glass ceramic compact, in the firing process. The resulting glass ceramic multilayered circuit board, after removal of the powdered layers, may have a large degree of surface roughness.
In a method disclosed in Japanese Unexamined Patent Application Publication No. 9-266363, a laminate of glass ceramic layers and alumina layers is fired to sinter only the glass ceramic layers so that the glass component contained in the glass ceramic layers penetrates unsintered alumina layers to bind the alumina layers. The glass component in the glass ceramic layers, however, does not penetrate the entity of the alumina layers in this method. Therefore, unbounded portions of the alumina layers are removed and the surfaces are polished before conductive films for circuit patterns are formed.
Although the surface roughness is reduced by the removing and polishing steps in this method, a removing step is required and conductive films cannot be formed on surfaces of the circuit board by simultaneous firing together with the glass ceramic layer.
In a method disclosed in Japanese Unexamined Patent Application Publication No. 5-136572, green sheets which are not sintered at a sintering temperature of a glass ceramic compact, are stacked on one side or two sides of the glass ceramic compact so as to sinter only the glass ceramic compact. Resin is loaded into powdered layers of the unsintered green sheets. This method does not require a step for removing the unsintered powdered layers, but does require a step for loading the powder into the unsintered powdered layers.
Accordingly, it is an object of the present invention to provide a composite laminate exhibiting reduced transverse shrinkage and high dimensional accuracy.
It is another object of the present invention to provide a method for manufacturing the composite laminate which does not require subsequent steps, such as a removing step and a resin-loading step, after a firing step.
According to a first aspect of the present invention, a composite laminate comprises first sheet layers including a first particulate aggregate and second sheet layers including a second particulate aggregate. Each internal second sheet layers is disposed between two first sheet layers and two of the second sheet layers are external constitute two main faces of the composite laminate. The thickness of the internal second sheet layers is greater than the thickness of the external second sheet layers. The first sheet layers and the second sheet layers are bonded to each other by penetration of a part of the first particulate aggregate contained in the first sheet layers into the second sheet layers.
Preferably, the thickness of the internal second sheet layers is about 1.75 to 2.67 times the thickness of the external second sheet layers.
Preferably, the first sheet layers have substantially the same thickness.
Preferably, the first particulate aggregate contains glass and the second particulate aggregate contains powdered ceramic.
The composite laminate may further comprises a conductive film at one of the interior and the exterior thereof, wherein the first sheet layers, the second sheet layers, and the conductive film constitute a circuit board.
The composite laminate may further comprises a cavity having an opening at least at one main face thereof.
According to a second aspect of the present invention, a method for manufacturing a composite laminate comprises a first step of preparing a green composite laminate including first green sheet layers containing a first particulate aggregate and second green sheet layers containing a second particulate aggregate unsinterable at a temperature for melting at least a part of the first particulate aggregate, wherein each second green sheet layer is disposed between two first green sheet layers, two of the second sheet layers constitute two main faces of the green composite laminate, and the thickness of the second sheet layers laminated in the interior of the green composite laminate is greater than the thickness of the second sheet layers disposed on the main face of the green composite laminate; and a second step of firing the green composite laminate at a temperature capable of melting a part of the first particulate aggregate and incapable of sintering the second particulate aggregate so that the part of the first particulate aggregate contained in the first green sheets is melted and penetrates the second green sheet layers to bond the first green sheet layers and the second sheet layers.
Preferably, the thickness of the second sheet layers laminated in the interior of the green composite laminate is about 1.75 to 2.67 times the thickness of the second sheet layers on the two main faces of the green composite laminate.
Preferably, the first step comprises a first sub-step of forming each of the second green sheet layers on each of the first green sheet layers to form a plurality of first green composite stocks, a second sub-step of laminating the plurality of first green composite stocks to form a plurality of second green composite stocks so that the two first green sheets come into contact with each other, and a third sub-step of laminating the plurality of second green composite stocks so that the two second green sheets come into contact with each other.
Alternatively, the first step comprises a first sub-step of forming each of the first green sheet layers on each of the second green sheet layers to form a plurality of first green composite stocks, a second sub-step of laminating the plurality of first green composite stocks to form a plurality of second green composite stocks so that the two second green sheets come into contact with each other, and a third sub-step of laminating the plurality of second green composite stocks so that the two first green sheets come into contact with each other.
Preferably, the thicknesses of the first green sheet layers are substantially the same in the first step.
Preferably, the first particulate aggregate comprises glass as a primary component and the second particulate aggregate comprises powdered ceramic as a primary component.
According to a third aspect of the present invention, a method for manufacturing a composite laminate comprises a first step comprising the sub-steps of preparing a first particulate aggregate, preparing a second particulate aggregate which is unsinterable at a temperature for melting at least a part of the first particulate aggregate, forming first green sheets containing the first particulate aggregate, forming each of second green sheets containing the second particulate aggregate on each of the first green sheets to form a plurality of first green composite stocks, and laminating the plurality of first green composite stocks to form a green composite laminate so that the two adjacent first green sheets form each of first green sheet layers and the two adjacent second green sheets form each of second green sheet layers; and a second step of firing the green composite laminate at a temperature capable of melting a part of the first particulate aggregate and incapable of sintering the second particulate aggregate so that the part of the first particulate aggregate contained in the first green sheet layers is melted and penetrates the second green sheet layers to bond the first green sheet layers and the second sheet layers.
Preferably, the first green sheet is resistant to a solvent contained in a slurry which is used for forming the second green sheet in the first step.
Preferably, the thicknesses of the first green sheet layers are substantially the same in the first step.
Preferably, the first particulate aggregate comprises glass as a primary component and the second particulate aggregate comprises powdered ceramic as a primary component.
According to a fourth aspect of the present invention, a method for manufacturing a composite laminate comprises a first step comprising the sub-steps of preparing a first particulate aggregate, preparing a second particulate aggregate which is unsinterable at a temperature for melting at least a part of the first particulate aggregate, forming second green sheets containing the second particulate aggregate, forming each of first green sheets containing the first particulate aggregate on each of the second green sheets to form a plurality of first green composite stocks, and laminating the plurality of first green composite stocks to form a green composite laminate so that the two adjacent first green sheets form each of first green sheet layers and the two adjacent second green sheets form each of second green sheet layers; and a second step of firing the green composite laminate at a temperature capable of melting a part of the first particulate aggregate and incapable of sintering the second particulate aggregate so that the part of the first particulate aggregate contained in the first green sheet layers is melted and penetrates the second green sheet layers to bond the first green sheet layers and the second sheet layers.
Preferably, the second green sheet is resistant to a solvent contained in a slurry which is used for forming the first green sheet in the first step.
Preferably, the thicknesses of the first green sheet layers are substantially the same in the first step.
Preferably, the first particulate aggregate comprises glass as a primary component and the second particulate aggregate comprises powdered ceramic as a primary component.
The composite laminate of the present invention exhibits reduced transverse shrinkage and high dimensional accuracy. Moreover, the composite laminate after the firing step can be used without additional steps, such as a removal step and a resin-loading step.
When the first sheet layers have substantially the same thickness, the first sheet layers have substantially the same transverse shrinkage in the firing step, and the second sheet layers suppress the transverse shrinkage. Thus, warping and distortion due to transverse shrinkage are suppressed.
Since the method in accordance with the present invention does not include the removal step of the second sheet layers and the resin-loading step, conductive films formed on the composite laminate and the composite laminate can be simultaneously fired.
In particular, the transverse shrinkage and dimensional variation readily occur in conventional composite laminates having cavities. The composite laminate of the present invention, however, exhibits reduced transverse shrinkage and dimensional variations even when the composite laminate has a cavity due to the above configuration.
The second particulate aggregate contained in the second sheet layer may have any property, for example, insulating, dielectric, piezoelectric or magnetic property. Thus, the resulting composite laminate can exhibit a specific electromagnetic property. An appropriate combination of these properties can produce, for example, an L-C-R composite substrate. When a high-wear-resistance, high-toughness second particulate aggregate is used, the composite laminate can have high mechanical strength. When a light-reflective or IR-reflective second particulate aggregate is used, the composite laminate can have a specific optical function.